Magnetic induction accelerator



Feb. 5, 1946- w. F. WESTENDORP MAGNETIC INDUCTION ACCELERATOR Filed June 17, 1942 3 Sheets-Sheet 1 Inventor: Wl' I lem F Westendorp,

H is Attorn ey.

Feb. 5, 1946. w; F. WESTENDORP 2,394,071

MAGNETIC INDUCTION ACCELERATOR Filed June 17, 1942 3 Sheets-Sheet 2 Fig.2.

Inventor: WE l lem F. Westendorp,

His Attorn e y.

1946. w. F. WESTENDORP MAGNETIC INDUCTION ACCELERATOR Filed June 17, 1942 3 Sheets-Sheet 3 m :n Fe mawm e O a? VP A Tmm W Patented Feb. 5, 1946 FFlCE MAGNETIC INDUCTION ACCELERATOR Willem F. Westendorp, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 17, 1942, Serial No. 447,372

7 Claims.

The present invention relates to apparatus for accelerating charged particles, such as electrons, by means of magnetic induction effects and is especially useful in connection with apparatus of the general character described in United States Patent No. 2,297,305, patented September 29, 1942, in the name of Donald W. Kerst, said patent being assigned to the General Electric Company, a corporation of New York.

Apparatus of the character referred to typically includes a closed vessel and a magnetic system for producing a time-varying magnetic field of such space distribution as to confine charged particles projected within the vessel to a closed orbit along which th particles are continuously accelerated as the field increases in magnitude. When the particles have been accelerated to a desired velocity, they are diverted from the accelerating orbit and used for the production of useful biological or other efiects.

In the operation of magnetic induction apparatus of the type specified, a major problem consists in the provision of suitable means for introducing charged particles into the orbital path in which acceleration is to occur, this problem being mainly one of correlating the action of the in- ;iecting means with the variations of the magnetic eld.

It is a principal object of the present invention to provide improved means by which this correlation may be achieved.

The time-varying magnetic field upon which the acceleration of particles depends is conveniently provided by the action of a cyclically varying electrical current and when it is so produced, acceleration in the desired direction occurs only during alternate half-cycles. Under these conditions, most satisfactory operation is realized it the particles to be accelerated are introduced into the accelerating orbit only at times when the magnetic field is starting to build up in the direction favorable to the desired acceleration. Broadly, this result is obtained in the present invention by providing in connection with the magnetic structure by which the accelerating field is produced a magnetic path which is adapted to change abruptly and reversibly from a saturated to an unsaturated condition in accordance with the variations of the magnetic field. By linkin an appropriate circuit with such a magnetic path it is possible to obtain sharp pulses of voltage which hear an exactly timed relation to the variations of the magnetic field and which may be used to assure the energizatlon of the particleinjecting means only at desired moments.

The features of the invention desired to be protected herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which Fig. 1 is a partially sectionalized elevation of an accelerator suitably embodying the invention; F18. 2 is a cross section taken on line 2-2 of Fig. 1; Fig. 3 is a schematic diagram illustrating means for energizing the magnetic structure of Fig. 1; Figs. 4 and 5 are enlarged views of certain electrode structures shown in Fig. 1; Fig. 6 is an enlarged detail view illustrating a feature of the construction of Fig. 1 with which the present invention is primarily concerned; Fig. 7 is a graphical representation useful in explainin the invention, and Fig. 8 is a circuit diagram illustrating the electrical interrelation of the various components embodied in the invention.

Referring particularly to Fig. 1, there is shown in section a closed, rotationally symmetrical glass vessel it) which defines within its interior anannular chamber I I. As will be explained in greater detail at a later point, the vessel I0 provides a circular orbit in which electrons may be accelerated to a high energy, say on the order of several million electron volts. The vessel is preferably highly evacuated and a. high resistance coating, such as an extremely thin layer Of silver (not shown) may advantageously be applied to the interior surface of the vessel to prevent wallh r ing.

The accelerating mechanism comprises a magnetic structure having generally circular pole pieces M and I5 which are coaxial with the annular vessel in. These pole pieces are constituted of laminated iron and have central portions ii and i8, respectively, which are of essentially planar character. Near their outer edges the poles are of tapered configuration, as indicated at 20 and 2|. A second or reverse taper is provided adjacent the periphery of each pole piece (1. e. at 22 and 23) for the purpose of providing a desired shaping of the marginal field. For decreasing the reluctance of the path between the opposed pole faces ii and i8 there is provided an insert in the form of two laminated iron disks 25 which are spaced from one another and from the other elements of the magnetic structure by insulating spacers 28. An externally closed magnetic circuit between the base portions of the pole pieces is provided by a rectangular iron core 9.

operation of the apparatus.

The magnetic structure is excited by means of a pair of series-connected coils 32 and 33 which surround the pole pieces i4 and i5 and which are energized in such a manner as to provide a cyclically varying fiux in the magnetic circuit. The energizing means may appropriately be of the character shown in Fig. 3, which illustrates diagrammatically a portion of the structure of Fig. 1

The coils 32 and 33 are shown connected in series with one another and with a condenser 35 which is assumed to be of such capacity as to resonate with the inductance of the coils at a frequency corresponding to the desired frequency of (This may be, for example, on the order of six hundred cycles per second although frequencies dillering widely from this value are also usable.) To supply the losses of the resonant circuit thus formed the coils 32 and 33 may be coupled to primary coils 31 and 38 which are directly energized from an A. C. power source 39 of the desired frequency. A relatively small amount of power supplied by the source 39 will serve to maintain the resonant system in excited condition.

Within the closed vessel (Fig. l) and also within the region of influence of the magnetic field produced by the pole pieces 64 and there is provided a thermionic cathode 40 which, in connection with associated electrodes 4| and 42 (Figs. 4 and 5), serves to generate an intermittent stream of electrons. The electrodes 40 to 42 are supported by a stem t3 and are supplied with potential and with heating current (in case of the cathode i0) by lead-in wire 4'8 sealed into the stem.

The electrons provided by the electrodes 40 to M are affected by the magnetic field in two ways. In the first place, since the field is in a direction transverse to the plane of the electron motion, it tends to cause the electrons to follow an inwardly spiralling path. Secondly, the time-varying flux enclosed by the orbit of any particular electron necessarily produces an electric field tending to accelerate the electron. In this latter respect, the apparatus as a whole consists essentially of a transformer with a secondary comprising a circular path along which the various electrons are accelerated. Although, in general, the voltage per turn in such a transformer is low, the electrons can achieve very high energies (e. g. several million electron volts) because of the tremendous number of turns which they execute during a single cycle of the magnetic flux variation.

It has been shown that by a proper design of the magnetic structure the centripetal force produced by the magnetic field existing at a particular orbit within the vessel in may be caused to balance the centrifugal tendencies of the accelerated electrons. In general, this result requires that the following relationship be satisfied:

where o is the flux included within the electron orbit, r is the radius of the aforesaid particular orbit, and Hr is the field strength at the orbit. This equation obviously means that the flux must be twice as strong as that which would be produced by a homogeneous field equal to the field Hf extending over the entire area enclosed by the orbital electron path.

The condition just specified may be realized by makin the reluctance of the magnetic path greater by an appropriate amount at the particular orbit than its average reluctance within the orbit. In order to maintain fixed proportionality between the enclosed flux and the guide field (i. e. the field Hr) at all times during the accelerating period, one may include in the magnetic path an air gap or its equivalent. It is readily practicable to control the dimension of such a gap from point to point over the pole area in such a fashion as to effect the balanced relation of guide field and enclosed flux which is desired for the purpose specified above and which is further necessary for radial and axial stability of the electron orbit. This may be done, for example, by a construction such as that shown in Fig. 1 in which the pole faces are outwardly tapered. (The reverse taper indicated at 22 and 23 is for the purpose of controlling the edge flux and does not produce any actual increase in flux near the outer edge of the pole faces.)

When all the conditions specified in the aforegoing are fulfilled, electrons introduced into the chamber I0 may be expected to be drawn into the particular orbit in which a balance of centripetal and centrifugal forces exists and to be continuously accelerated along such orbit as long as the magnetic field increases in value. Assuming that the peak value of the magnetic field is sumciently high, a total energy on the order of several million electron volts may be acquired by the accelerated electrons in a small fraction of a second. (In a particular construction, it has been calculated that each orbital gyration of a given electron produces an increase in its energy of 70 electron volts, and that as many as 400,000 gyrations may be completed within the period'of a single accelerating cycle.)

It is, of course, desirable to provide some practical way in which the energy of the fully accelerated electrons may be effectively utilized. In the arrangement of Fig. 1 this is accomplished by so arranging matters that while the normal accelerating orbit of the electrons is inside the space occupied by the injecting electrode assembly, the electron crbit may be expanded to causetne electrons to imp nge on the exposed parts of the assembly as the end of the acceleratin cycle is approached. The resultant impact of the accelerated electrons (e. g. on the target comprised by the exposed surface of the electrode 4|) will produce X-rays of an intensity corresponding to the velocity of the electrons involved. Expansion of the electron orbit may be obtained in one way by means of auxiliary coils provided in connection with the pole pieces l4 and I5 and adapted to be energized in such fashion as to disturb the normally balanced condition of the magnetic field. Such auxiliary coils are indicated at 46 and 56 in Fig. 1. They are more fully described and claimed in D. W. Kerst application, Serial No. 445,465, filed June 2, 1942.

The present invention is primarily concerned with the provision of suitable means for injecting electrons into the accelerating chamber at a proper time during the cycle of variation of the magnetic field by which acceleration effects are to be obtained. It has been found desirable in this connection to assure that the electrons shall be introduced with a substantial initial velocity, say a velocity corresponding to an acceleration of several thousand (for example, twenty thousand) volts. Moreover, in order to assure that a significant portion of the injected electrons shall be drawn into the accelerating orbit, it is necessary that their injection shall occur at a time when the magnitude of the magnetic field is appropriate to effect the "capture of electrons of the given velocity. This means that the time of energization of the injecting means must, be accurately correlated with the variations of the magnetic field.

The desired correlation is accomplished in accordance with the present invention by the use of means linked with the main magnetic structure and providing a magnetic path which is adapted to change abruptly from a saturated to an unsaturated condition and thus to produce abrupt pulses of voltage which may be employed to cause equally abrupt energization of the electron injecting means. In the construction of Fig. 1, the means employed for this purpose comprises a thin strip 50 of ferromagnetic material (e. g. Permalloy, or some other high permeability *metal) Which is bridged between the pole pieces l4 and I5. This strip, which is shown in enlarged detail in Fig. 6 becomes mag-V netically saturated when the magnetic field existing between the pole pieces is still of relatively low value. Accordingly, the variations of magnetic flux in the strip are as indicated by curve A in Fig. 7 in which the cyclical character of the graph corresponds to the cyclical variations of the main magnetic field. Due to the ready saturability of the strip, it is only at points near a region of reversal of the magnetic field, as indicated at a, b, 0, etc., that any substantial change in magnetic flux through the strip can occur. At intermediate points the strip is fully saturated and its flux is substantially constant.

In order to make use of this characteristic of the strip 50, there is provided in connection with it a coil 5i which is wound on the strip and which, as will be further explained at a later point, serves to control the energization of electron-injecting means for the vessel ill. The voltage induced in the coil 5! is in the form of a series of pulses e (curve B of Fig. 7) whose spacing in time corresponds to the spacing of the flux reversals a, b, through strip 50.

For the purpose of controlling the time of occurrence of the voltage peaks 6 developed in the coil a further coil 52 (Fig. 6) is provided on the strip 50 and is adapted to be connected to a unidirectional power source (see Fig. 8). By this means the time of occurrence of saturation of the strip 50, as controlled by the variations of the magnetic field between the pole pieces M and I5, can be advanced or retarded to some degree by the use of a magnetic bias produced by the coil 52. For present purposes, the bias imposed is preferably such as to oppose the main magnetic field as it increases in a sense favorable to accelerating electrons in the desired direction of acceleration. This makes it possible to delay the injection of electrons until the main magnetic field has attained a value great enough to balance the centrifugal tendencies of electrons introduced at a substantial velocity.

The utility of the arrangement described in the foregoing may best be understood by consideration of Fig. 8 which represents diagrammatically a suitable energizing system for the injecting electrodes 40, M and 62. (In the circuit illustrated the electrodes 4! and 42 are connected together, although it may be preferable in some cases to provide a negative bias for the electrode 6i Voltage is impressed between the cathode Ml and the electrode 4! by connecting these electrodes across a resistor 60 which is intermittently supplied with current in a manner now to be described.

The primary power supply comprises an A.-C. line indicated at El. This is connected through an autotransformer 82 and a step-up transformer B3 to a high voltage rectifier 54. In circuit with the rectifier and with the resistor 60 there is provided a condenser 66 which is adapted to be charged through the rectifier, the rate of charging being governed by a current limiting resistor 61.

In parallel with the condenser 66 and the resistor 60 there is provided a three-element discharge device 10 which is of a discontinuously conductive type (e. g. a thyratron). The anode 'II of this device is connected directly to the positively charged terminal of the condenser 66 and the cathode I2 is connected to a terminal of the resistor 60 so that whenever the tube 10 becomes conductive the condenser 66 is, in eifect, connected directly across the resistor.

In order that the tube I0 may be rendered conductive only at desired intervals, its control grid 13 is connected through a blocking condenser 14 I to the coil 5|, thi being a coil wound directly on the saturable strip 50 of Fig. 1 as previously explained (a grid resistor 15 and negative bias source 15 are also provided in connection with the grid 13). As a result of this arrangement, whenever the strip 50 shifts from saturated to unsaturated condition, the voltage pulse which is induced in the coil 5| is impressed on the grid 13 and, assuming that the pulse is of a discharge-favoring polarity. breakdown of the tube 10 occurs. As has been previously suggested, the timing of the triggering pulse may be controlled to some degree by the use of the biasing coil 52, which is shown connected in series with a D.-C. source I6 through a variable resistor 11 and a fixed reactor 18. The resistor has the function of regulating the D.-C. biasing current flowing through the coil 52, while the reactor 18 prevents the D.-C. circuit from loading the coil 5| during the peaking period. With this arrangement, it is possible to cause the tube 10 to discharge and thus to apply voltage between the injecting electrodes 40 and 42 at a time which is so correlated to the variations of the main magnetic field as to assure that at least a considerable portion of the resultant electrons shall be drawn into and retained in the desired accelerating orbit. In general, injection of electrons should occur shortly after the magnetic field has begun to build up in a direction favorable to accelerating the injected electrons in the desired direction around the accelerating chamber.

It will be understood that the illustrated configuration of the saturable strip 50 is exemplary and not limiting and that many variant forms may be employed. I aim in the appended claim to cover all such equivalent variations of structure or use as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. A magnetic induction accelerator includin a chamber within which charged particles may follow an orbital path, means including spaced pole Dieces for producing a varying magnetic field of such space distribution as to confine particles within the chamber to a desired orbit while such particles are being accelerated along said orbit. means effective when energized to inject charged particles within said chamber, means magnetical- 1y linked with said pole pieces and defining a magnetic path which is adapted to change from saturated to unsaturated condition in accordance with the variations of said magnetic field, and means responsive to such change in condition of said magnetic path for producing abrupt intermittent energization of said injecting means.

'2. A magnetic induction accelerator including a chamber within which charged particles may follow an orbital path, means including spaced magnetic pole pieces adjacent to said chamber for producing a varying magnetic field of such space distribution as to confine particles within the chamber to a desired orbit while such particles are being accelerated along said orbit, means effective when ener ized to inject charged particles within said chamber, means bridging the space between said pole pieces and providing a magnetic path adapted to change from saturated to unsaturated condition in accordance with the variations of said magnetic field, and circuit means linked with said magnetic path for controlling the energization of said injecting means.

3. A magnetic induction accelerator including means defining a rotationally symmetrical chamber within which charged particles may follow an orbital path, a pair of spaced pole pieces coaxial with said chamber, means for producing a cyclically varying magnetic field between the said pole pieces, the space distribution of the said magnetic field being such as to confine particles within the chamber to a fixed orbit while they are being continuously accelerated along the said orbit, means effective when energized to inject charged particles within the chamber, a body of magnetic material bridging the space between the said pole pieces and providing a magnetic path capable of abrupt changes from saturated to unsaturated condition, circuit means magnetically linked with the said body of material, and means in circuit with said last-named means for abruptly energizing the said injecting means in response to changes in the condition of saturation of the said magnetic path.

4. A magnetic induction accelerator including a chamber within which electrons may follow an orbital path, means including spaced pole pieces adjacent to said chamber for producing a cyclically varying magnetic field of such space distribution as normally to confine electrons within the chamber to a fixed orbit while continuously accelerating them along said orbit, electrodes in said chamber and effective when energized to inject electrons within the chamber, a potential source adapted to be connected to said electrodes to energize the same, a strip of ferromagnetic material connected between the said pole pieces and providing a magnetic path capable of abrupt changes from saturated to unsaturated condition, and circuit means linkin the said path and efiective in response to a change in the condition thereof to connect the said potential source to the said electrodes.

5. A magnetic induction accelerator including a chamber within which charged particles may follow an orbital path, means including spaced magnetic pole pieces for producing a cyclically varying magnetic field of such space distribution as to confine particles within the chamber to a desired orbit while continuously accelerating them along said orbit, means eflective when energized to inject charged particles within said chamber, means associated with said pole pieces and defining a magnetic path which is adapted to change cyclically from saturated to unsaturated condition in accordance with the variations of said magnetic field, means responsive to such changesin condition of said magnetic path for producing abrupt intermittent ,energization of said injecting means, and means for imparting a magnetic bias to said magnetic path so as to control the time of energization of said injecting means.

I 6. A magnetic induction accelerator including a chamber within which electrons may follow an orbital path, means including spaced magnetic pole pieces for producing a cyclically varying magnetic field of such space distribution as to confine electrons within the chamber to a fixed orbit while continuously accelerating them along said orbit, electrodes in said chamber and'effective when energized to inject electrons within the chamber, a strip of ferromagnetic material connected between said pole pieces and capable of abrupt changes from saturated to unsaturated condition, means including a first winding linking the said strip and effective in response to a change in the condition of the strip to produce energization of the said electrodes, and means including a second winding linking the said strip for imparting a magnetic bias to the strip so as to control the time of ,energization of the said electrodes.

'7. A magnetic induction accelerator including an evacuated annular chamber within which electrons may follow orbital paths, means including magnetic pole pieces spaced adjacent to said chamber to produce a cyclically varying magnetic field of such space distribution as to confine electrons within said chamber to a substantialiy circular orbit while continuously accelerating them along said orbit, a thermionic electron source effective when energized to introduce electrons into said chamber, a. member of magnetic material magnetically linked to said pole pieces and providing a magnetic path adapted to change cyclically and abruptly from saturated to unsaturated condition in accordance with the variation of said magnetic field, circuit means including an electronic trigger device linked with said magnetic path for controlling the operation of said electron source and a target upon which said electrons may impinge when accelerated.

WILLEM F. .WESTENDORP. 

